Two microphone stethoscope chestpiece constructions are well known and generally have a tubular stem or shaft secured in a body and rotatably alignable with a selected one of the microphones (e.g., a conventional bell microphone for low frequency heart sounds or a conventional diaphragm microphone for high frequency lung sounds). Typically the shaft has a spring loaded detent to hold the shaft in alignment with the selected microphone while blocking sound transmission into the stem from the microphone not in use. Manufacture of chestpieces with indexing detents is very expensive because of multiple machining steps to mate the various parts, complex assembly, and extensive testing to assure that no sound leaks into the stem from pathways other than the selected microphone.
U.S. Pat. No. 3,224,526 to Weber describes an indexing detent that has been used for many years with cardiology stethoscopes (having a relatively large stem). The indexing detent is comprised of a coil spring lying transversely of the stem and wholly within the stem and a stop. The assembly of the coil spring and stop aligns with two openings in the wall of the stem. The stop is preferably a ball bearing carefully machined to seat in an opening at the apex of the microphone not in use. Sound enters the stem from the microphone in use through a hole in the microphone at its apex that is aligned with the hole in the stem opposite the stop. When pressure is applied to the stem to rotate it, the ball bearing is biased against the spring inside the stem and rotates within the stem. When proper alignment is reached, the compression on the spring is released and the ball bearing is forced against the opening to the microphone not in use and prevents air leakage. While stethoscopes carefully manufactured according to this design work very well over a long period of time, the manufacturing cost is high. The ball bearing and the openings from each of the microphones must be carefully machined to mate and they must be individually tested for second leaks across the ball bearing-microphone seal. Additionally, any wear on the microphone opening or the ball bearing (e.g., from foreign matter) can create sound leaks.
U.S. Pat. No. 3,152,659 to Littman describes another indexing detent that has performed very well for many years in stethoscopes having smaller stems than cardiology stethoscopes. The body of the chestpieces has two diametrically opposed microphones and a central bore. Each microphone has an opening into the central bore. The openings are aligned across the bore. The bore is machined to have two grooves that are spaced normal to the openings. A U-spring is fit within the end of the stem. The U-spring has enlarged bight sections which resiliently engage the grooves in the bore. The position of the U-spring may be locked with respect to the stem by providing short slits along the end of the stem through which the enlarged bight sections of the U-spring extend. Manufacture of this chestpiece requires very close tolerances to assure that the grooves are normal to the openings from the microphones, that the slits in the stem are normal to the opening in the stem, that the U-spring mates with the grooves without allowing the stem to wobble and with appropriate resiliency to allow detent indexing. While stethoscopes made with this design perform very well, maintenance is required if the U-spring breaks. Eventually the grooves in the bore wear so that the U-spring does not securely hold the hole in the stem in alignment with the selected opening, requiring replacement of the entire chestpiece.